1. Field of the Invention
This present application relates to an optical gas sensor. The application moreover relates to a method for optically measuring the presence of a gas using the gas sensor and to a gas sensing system.
2. Description of the Related Art
Surface Plasmon Polaritons (SPP) or Surface Plasmons are electromagnetic waves that can propagate along an interface between two media with dielectric constants of opposite sign, e.g. at the interface between a metal and a dielectric material. They correspond to fluctuations in the electron density at the interface between both materials, for example excited by photons. Typically Surface Plasmon Resonance (SPR) is observed by measuring the reflectivity of p-polarized light of a given wavelength at a metal-dielectric interface for a varying angle of incidence. Alternatively, the angle of incidence is fixed and the wavelength is varied. This is possible by e.g. using the so-called Kretschmann configuration, illustrated in FIG. 1.
In the Kretschmann configuration, a high refractive index prism 10 is placed on a stack of a dielectric layer 12 and a metal film 11 (such as, e.g., a gold film). When light 21 propagating in the prism is incident on the metal film 11 at a large angle of incidence a, it is totally internally reflected, which means that all the light is reflected. There is, however, an evanescent transmitted field 23 that can penetrate the metal film. If the metal film 11 is sufficiently thin, e.g. with a thickness less than 100 nm for visible light, this evanescent field 23 can couple to SPPs on the gold-dielectric interface. When the incident light 21 couples to SPPs the intensity of the reflected light 22 is reduced and results in Attenuated Total Reflection (ATR). The coupling conditions to SPPs depend on the refractive index of the dielectric layer 12. If the refractive index of the dielectric layer 12 changes, the coupling conditions between a light wave and the SPP also change. These include changes are in the angle of excitation of SPPs (α in FIG. 1), the intensity of the reflected light 22, and the wavelength of excitation of SPPs at a specific angle. It is possible to excite SPPs with different wavelengths of light by changing the angle of incidence.
Since the SPPs propagate at the interface between the metal and the dielectric layer, they are very sensitive to changes in this interface. The principle of chemical sensing using SPR involves placing a thin layer of chemically active material (sensing layer, layer 12 in FIG. 1) on the metal surface along which the surface plasmons propagate. The electric and magnetic field of SPPs decay exponentially in the direction perpendicular to the interface. This confinement of the electromagnetic field to the interface is the reason why SPPs can be used as a sensitive tool to detect changes in surface properties. Changes in the sensing layer 12 brought about by the presence of an analyte result in changes in the SPR, which can be monitored, allowing a determination of the analyte concentration. A typical experimentally obtainable resolution of the refractive index of the sensing layer using this method is in the order of 2×10−6. Several improvements of the basic SPR technique have been proposed by using improved read-out techniques. Changes in refractive index as small as 10−7 have been experimentally measured. However, these techniques are not suited for handheld, low power devices as they require a complicated and bulky read-out apparatus.
Several gas sensors based on SPR techniques have also been reported in literature. Typically, the sensing layer is based on redox-active molecules that form charge transfer complexes in the presence of electron-withdrawing and/or electron-donating gases. NO2 is a typical example of such a gas. Phthalocyanines have mainly been used as sensing layers, although porphyrins and their derivatives (or other molecules that form charge transfer complexes) may also be used. Selectivity and sensitivity to certain gases may be achieved by incorporation of side groups or metal ions in the sensing molecules. The formation of charge transfer complexes results in changes of the optical properties of the sensing layer that can be detected with a SPR-based technique.
For example, in “Surface plasmon polariton studies of 18-crown-6 metal-free phthalocyanine”, M. J. Jory et al, J. Phys. D: Appl. Phys. 27 (1994) 169-174, SPR in the Kretschmann configuration is used to characterize the response (changes in optical permittivity) of a 14 nm thin film of 18-crown-6 metal-free phthalocyanine upon exposure to 100 ppm NO2 in air. Reflectivity was measured versus the angle of incidence for light of different wavelengths.
In “A surface-plasmon-based optical sensor using acousto-optics”, Meas. Sci. Technol. 6 (1995) 1193-1200, a wavelength-tunable optical SPR sensor with a fixed angle of incidence, incorporating an acousto-optic tunable filter (AOTF) is described. The AOTF is used to control the wavelength of a p-polarized light beam incident on a gold-coated diffraction grating. A SPR is observed as a deep minimum in the intensity of the reflected beam as the incident wavelength is varied. By modulation of the AOTF, combined with lock-in techniques, the SPR minimum position can be measured with a precision of 0.0005 nm. The sensitivity of this system was found to be equal to a change in the refractive index of a gas of 1×10−6. By adding a chemically active overlayer (18-crown-6 metal-free phthalocyanine) to the system a concentration of 0.01 ppm NO2 in N2 was detected with a response time of about 15 minutes. Although this system provides an improvement as compared to conventional angle-scan systems because the need for a bulky prism and an angle-scan system is avoided, it still requires wavelength modulation by an external AOTF and involves a relatively complicated lock-in technique in order to reach sub-ppm detection levels.
Localized surface plasmon polaritons (LSPP) are surface plasmon polaritons that are localized at the surface of nanoparticles. They exhibit relatively broad resonances compared to SPP, however, they have attracted interest for sensing because they cause large field enhancements in the direct environment of the nanoparticles. This allows them to detect the presence of, for example, (bio)molecules in very small samples as described for example in “A unified view of propagating and localized surface plasmon resonance biosensors”, Amanda J. Haes and Richard P. Van Duyne, Anal Bioanal Chem (2004) 379: 920-930. However such sensors based on LSPP show resonances that are much broader than those of SPP-based sensors.
Periodic arrays of metal nanoparticles show sharp resonance peaks in their far field extinction upon irradiation with light. These resonance peaks are called surface lattice resonances and are caused by long range diffractive coupling of localized surface plasmon resonances of the nanoparticles in a periodical array. Long range coupling between the nanoparticles, and therefore surface lattice resonances, are possible when the nanoparticles are sandwiched between dielectric layers with similar refractive index (optically symmetric layers). If there is a large mismatch between the refractive index of the substrate and the surroundings of the nanoparticles, long range coupling is weaker. The spectral position of these resonance peaks is determined by geometrical factors, e.g. the pitch of the nanoparticle array and the shape and size of the individual nanoparticles, and material factors, e.g. the material of which the nanoparticles are formed as well as the refractive index of their surroundings. An example of a gold nanoparticle array and its near field and far field scattering characteristics is reported in “Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays”, Y. Chu et al, Applied Physics Letters 93, 181108 (2008).